A micropuncture study now in progress has shown that in the rat proximal tubule, phosphate is reabsorbed primarily in the H2PO minus 4 form. The data suggest that an active transport process is responsible for this, although passive diffusion cannot be ruled out. HPO equals 4 is reabsorbed more slowly, most likely by a passive diffusion process. Parathyroid hormone appears to specifically block the transport of H2PO minus 4, while having little or no effect on HPO equals 4. In another project, is has been found that sodium reabsorption by the proximal tubule depends upon the presence of HCO minus 3 in the tubular lumen. When the tubule was perfused with isotonic NaCl, fluid adsorption was delayed for the first 1 - 1.5 mm of length, after which it increased in rate. In contrast, when a HCO minus 3 containing solution is used to perfuse the tubule, fluid absorption starts immediately and continues at a constant rate. These observations suggest that an Na ions -H ions exchange process occurs at the luminal membrane which is essential for the initial entry of Na ions into the cell. Inhibition of carbonic anhydrase has no effect on Na ions reabsorption when HCO minus 3 is not present in the lumen, but has a marked inhibitory effect when normal concentrations of HCO minus 3 are present. These experiments suggest that a chloride gradient is not an important driving force for fluid absorption in the proximal tubule.